The present invention concerns solenoid valves, also called electromagnetically controlled valves, comprising a housing defining a chamber provided with a fluid flow passage opening and in which a movable assembly is subjected to the action of a return spring biasing it towards a first end position and to the action of an electromagnetic coil whose action brings it to a second end position, against the action of the spring, when it is energized.
Solenoid valves of this type are known in the art. In most cases, one of the end positions is defined by abutment of a closure component of the movable assembly against a seat surrounding the opening.
Such solenoid valves are satisfactory as long as the passage opening has an appreciable cross-sectional area and as long as the components of the valve are sufficiently sized to avoid rapid wear in use.
On the other hand, for a number of uses, the cross-sectional area to be controlled is quite small and the overall size of the valve must be as small as possible. As an example, valves for metering a flow of liquid fuel in a fuel feed system for an internal combustion engine, in response to an operating parameter of the engine, typically the composition of the exhaust gases, should be of minimum size. It has already been suggested to meter the fuel with a solenoid valve which is not of the type providing a progressively variable opening, but is supplied with periodical electric pulses whose aperture ratio depends on the value of the operating parameter. The need to operate at a high frequency requires the movable assembly to have low inertia. If a fuel passage section is controlled directly, the flow cross sectional area when the valve is open must be very small.
Under these circumstances, there is fast wear and tear of the components of the valve, particularly the closure member and the seat.
Attempts have been made to overcome the difficulty by controlling the depression which draws fuel from a constant level chamber rather than the fuel passage cross section: the solenoid valve is then placed in an air circuit and may be larger in size. This solution is however only a palliative.
It is an object of the invention to provide an improved solenoid valve; it is a more specific object to provide a solenoid valve whose bulk and fluid passage section may be very small, having however a long operating life.
The invention makes use of the finding that, very often, it is not necessary for the solenoid valve to close the passage: in the example given above of a fuel supply system, fuel flow delivered to the engine is always greater than a predetermined minimum value, when the engine is operating.
According to an aspect of the invention, there is provided an solenoid valve having a movable assembly which comprises a armature whose abutment against guide means determines a second end position and a flow-control member fast with the armature, which defines with the opening a flow passage of maximum cross-sectional area when the assembly is in its first end position and a flow passage of minimum cross-sectional area different from zero when the assembly is in its second end position.
Such a valve, which may be termed "all or little" (in opposition to the "all or nothing" valves), may have a very extended life, for there is no substantial wear of the closure member. The abutment in the direction corresponding to the smallest flow cross section may be between parts of large size and may be through plane surfaces.
The closure member may be securely fixed to the armature, but it is then necessary to machine the armature and the closure member accurately, which results in a high cost. Moreover, heating of the armature during operation of the solenoid valve, particularly when the latter is controlled with rectangular pulses at high frequency, causes thermal expansions which may result in separation of the armature from the closure member. In an advantageous embodiment, the closure member is retained with the armature by a spring which holds it against a stop surface provided for this purpose on the armature.
The invention will be better understood from the description which follows of solenoid valves which form particular embodiments thereof. The description refers to the accompanying drawings.